Many of the parts used for vehicles are produced by casting and are internally subject to residual stress. That is, such a part is very likely to experience high residual stress due to shrinkage after casting and high residual stress greatly shortens the fatigue life of the part.
Cylinder blocks of an engine are typically made of aluminum material for lightness and better fuel consumption of a vehicle. However, cast iron material is still applied to an interior of a cylinder bore to prevent deterioration of the rigidity and/or fatigue life of a cylinder block. Such a cast iron material and aluminum have different coefficients of thermal expansion. Therefore, relatively high residual stress is formed at a siamese region where adjacent cylinder bores meet on the top of the cylinder block after casting of the cylinder block.
Residual stress formed at the siamese region may cause a crack so as to deteriorate fatigue strength and durability of an engine. Consequently, measurement of residual stress of a siamese region of a cylinder block is required during development of a new engine. When the residual stress is measured to be higher than a standard, the manufacturing method or structure of the cylinder block should be changed so as to lower the residual stress.
Residual stress is typically measured by a cutting or non-cutting method. A non-cutting method does not cause damage to a fabricated article since a crystalline structure may be analyzed using X-ray, ultrasonic wave, electromagnetic wave, or etc. According to a cutting method, the degree of residual stress is measured by measuring deformation of the article that is produced by removal of residual stress due to cutting of the article.
Among various schemes for such a cutting method, a drilling scheme and a full cutting scheme are most widely used. According to the drilling scheme, strain gauges are respectively attached to 0°, 120°, and 270° positions around a target area of measurement, in a generally circular arrangement. Subsequently, deformation, and accordingly residual stress, of the article is measured by the strain gauges positioned therearound, while the target area is being removed by a drill or laser.
On the other hand, according to a full cutting scheme, a strain gauge is attached at a target area of the residual stress measurement. Then, the target area is fully separated from the article by cutting the article along a full circumference around the strain gauge. A resultant deformation, and equivalently residual stress, is then measured.
Each of the above mentioned conventional methods have associated drawbacks. A drawback of the non-cutting method is that a very expensive and complicated apparatus is required. In addition thereto, since a cylinder block of an engine is very big in size for such an apparatus, such a non-cutting method is not believed to be appropriate for a measurement of residual stress of a cylinder block. A drawback of the drilling scheme includes the fact that a relatively wide area around the target area of the measurement should be secured to be attached with strain gauges. However, a siamese region of a cylinder block does not provide sufficient neighboring area, and accordingly, it is not a good example for an application of such a drilling scheme.
Therefore, according to a conventional scheme for measuring residual stress of a siamese region of an engine, a full cutting scheme is widely adopted, and accordingly, a target area of measurement attached with a strain gauge is fully separated from the article by cutting the article along a full circumference around the strain gauge. However, according to such a full cutting scheme, measurement of residual stress requires excessive time, since a siamese region should be fully separated from a cylinder block.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.